AUTHORITIES say 16-year-old Alex Hribal will be charged as an adult following a frenzied morning knife attack at a school near Pittsburgh which left more than 20 people injured.

Hribal appeared before Magisterial District Judge Charles Conway and was charged with four counts of attempted homicide and 21 counts of aggravated assault.

He was shackled by his hands and feet and was dressed in a hospital gown.

He allegedly stabbed 21 teenagers and a security guard in a bloody 30-minute rampage in the classrooms and hallways of Franklin Regional High School.

At least five students were critically injured in the shocking assault, Murrysville police chief Thomas Seefeld said.

The 16-year-old boy walked down a hallway flashing two eight-inch knives, stabbing fellow pupils and the guard as they began arriving for what should have been a routine school day shortly after 7am (9pm AEST), Mr Seefeld said.

Twenty-one students and one male security guard were injured, mostly in the chest and stomach, officials confirmed.

The school principal and another member of staff tried to overpower the suspect who was then arrested, Mr Seefeld said.

Taken to court ... Alex Hribal, the suspect in the stabbings at the Franklin Regional HigSource:AP

Interviews with nearly two dozen students yielded precious little background about Hribal. Many students said they did not know him.

Others, like sophomore Anissa Park, who knew Hribal from elementary school but hadn’t talked with him for some time, invariably used the words “shy” and “quiet” to describe him.

Some said he was involved in athletics, including street hockey, track and tennis, though The Pittsburgh Post-Gazette could not confirm those details.

Another student who knew Hribal, though he spoke on condition of anonymity, said he was stunned by the attack.

“I know him pretty well,” the boy said, adding that Hribal’s interests swung toward the usual for a teenage boy, including hockey and video games. “I’ve never seen any anger from him, ever. He never seemed like someone who would do anything violent. He never seems very upset or any of that.”

Though his parents could not be reached Wednesday, Hribal’s lawyer Patrick Thomassey said he had spoken to them about an hour before the teen’s arraignment.

“They did not foresee this coming. They expressed absolute horror,” Thomassey said, adding that the family’s thoughts were with the victims.

Thomassey said the teenager was not a loner nor was he aware of any instances of bullying that would have provoked the attack.

“He’s scared,” Thomassey said. “He’s a young kid. He’s 16, looking like he’s 12. This is all still new to him.”

Thomassey said Alex is a B-plus student from a stable home, describing his family as “like Ozzie and Harriet.”

“They have dinner together every night,” the attorney said, though he would not provide details of his discussions with his client.

District Attorney John Peck said during the brief hearing that Hribal made some statements after school officials tackled him that indicated he wanted to die.

photo galleryUS school stabbing

A Salvation Army disaster services vehicle drives past a school bus onto the campus of the Franklin Regional School District where several people were stabbed at Franklin Regional High School. Picture: AP Photo/Keith Srakocic1 of 10

Emergency responders gather in the parking lot of the high school on the campus where several people were stabbed at Franklin Regional High School on Wednesday, April 9, 2014, in Pittsburgh. Picture: AP Photo/Keith Srakocic2 of 10

A pair of women walk arm-in-arm off the campus of the Franklin Regional School District, where up to 20 people were stabbed. Pictuer: AP Photo/Keith Srakocic3 of 10

Students leave the campus of the Franklin Regional School District where several people were stabbed at Franklin Regional High School on Wednesday, April 9, 2014, . The suspect, a male student, was taken into custody and being questioned. Picture: AP4 of 10

Students walk down the street away from the campus of the Franklin Regional School. Picture: AP Photo/Gene Puskar5 of 10

Emergency responders gather at the high school on the campus of the Franklin Regional School District, where up to 20 people were stabbed. Picture: AP Photo/Tribune Review, Sean Stipp6 of 10

Forbes Hospital President Chief Medical Officer Dr. Mark Rubino, center, addresses the media as vice president and chief nursing Amber Egyund, left, and Forbes Hospital President and CEO Reese Jackson listen in during the second morning news conference at at the hospital Wednesday, April 9, 2014. Picture: AP Photo/Tribune Review, Evan Sanders7 of 10

An ambulance and emergency vehicles leave Franklin Regional High School where more then a dozen students were stabbed by a knife wielding suspect. Picture: AP Photo/Tribune Review, Brian F. Henry8 of 10

A student and guardian walk to their car from Franklin Regional Middle School after more then a dozen students were stabbed by a knife wielding suspect at nearby Franklin Regional High School on Wednesday, April 9, 2014. Picture: AP Photo/Tribune Review, Brian F. Henry9 of 10

Mike Kane, a parishioner at the Newlonsburg Presbyterian Church in Murrysville, Pa., works on a sign outside his church that abuts the Franklin Regional School District campus, where more then a dozen people were stabbed. Picture: AP Photo/Gene J. Puskar10 of 10

A Salvation Army disaster services vehicle drives past a school bus onto the campus of the Franklin Regional School District where several people were stabbed at Franklin Regional High School. Picture: AP Photo/Keith Srakocic

Emergency responders gather in the parking lot of the high school on the campus where several people were stabbed at Franklin Regional High School on Wednesday, April 9, 2014, in Pittsburgh. Picture: AP Photo/Keith Srakocic

A pair of women walk arm-in-arm off the campus of the Franklin Regional School District, where up to 20 people were stabbed. Pictuer: AP Photo/Keith Srakocic

Students leave the campus of the Franklin Regional School District where several people were stabbed at Franklin Regional High School on Wednesday, April 9, 2014, . The suspect, a male student, was taken into custody and being questioned. Picture: AP

Students walk down the street away from the campus of the Franklin Regional School. Picture: AP Photo/Gene Puskar

Emergency responders gather at the high school on the campus of the Franklin Regional School District, where up to 20 people were stabbed. Picture: AP Photo/Tribune Review, Sean Stipp

Forbes Hospital President Chief Medical Officer Dr. Mark Rubino, center, addresses the media as vice president and chief nursing Amber Egyund, left, and Forbes Hospital President and CEO Reese Jackson listen in during the second morning news conference at at the hospital Wednesday, April 9, 2014. Picture: AP Photo/Tribune Review, Evan Sanders

An ambulance and emergency vehicles leave Franklin Regional High School where more then a dozen students were stabbed by a knife wielding suspect. Picture: AP Photo/Tribune Review, Brian F. Henry

A student and guardian walk to their car from Franklin Regional Middle School after more then a dozen students were stabbed by a knife wielding suspect at nearby Franklin Regional High School on Wednesday, April 9, 2014. Picture: AP Photo/Tribune Review, Brian F. Henry

Mike Kane, a parishioner at the Newlonsburg Presbyterian Church in Murrysville, Pa., works on a sign outside his church that abuts the Franklin Regional School District campus, where more then a dozen people were stabbed. Picture: AP Photo/Gene J. Puskar

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Emergency response ... ambulances surround Franklin Regional High School after the stabbiSource:AP

Nate Moore, 15, said he saw the first attack and was going to try to break it up when the boy got up and slashed his face, requiring 11 stitches.

“It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead,” he said.

The attacker “had the same expression on his face that he has every day, which was the freakiest part,” Moore said.

“He wasn’t saying anything. He didn’t have any anger on his face. It was just a blank expression.”

To be charged as an adult ... Alex Hribal, the suspect in the stabbings at Franklin RegioSource:AP

Otther witnesses described chaotic scenes that unfolded during the attack.

“I was walking into the school and a stampede of people were running after me,” Kari Lee, a student at the school, told NBC News.

Another student said he saw “blood all over the floor” as he tried to flee the building.

Distraught ... a student and guardian walk to their car.Source:AP

Student Mia Meixner told CNN that she saw the assailant when she was at her locker.

“I saw the kid who was stabbing people get up and run away,” she said. “He was very quiet. He just was kind of doing it ... and he had this, like, look on his face that he was

just crazy and he was just running around just stabbing whoever was in his way.”

Nate Scimio is believed to be the young man who pulled the fire alarm, alerting students to the attack and encouraging them to run, the Daily Mail reports.

He is also believed to have been injured when he stepped in front of other students to protect them.

The teenager later posted a photo of himself wearing a hospital gown at UPMC Children’s Hospital.

Hero ... Nate Scimio reportedly pulled the fire alarm that first alerted students to theSource:Supplied

As for what set off the attack, Murrysville Police Chief Thomas Seefeld said investigators were looking into reports of a threatening phone call between the suspect and another student the night before.